Prosecutors are pressing their attack on questionable practices used by bond traders, appealing a judge's decision to toss out the conviction of a former Nomura Holdings Inc. managing director caught up in the sputtering U.S. crackdown.

Michael Gramins was found guilty last year of one conspiracy count after a trial in Hartford, Connecticut, and cleared of six fraud counts; jurors deadlocked on two other charges. Shortly before his retrial was to start, a federal appeals court in another case tossed out the conviction of another trader, Jesse Litvak — and that prompted Gramins's judge to follow suit.

Federal prosecutors on Thursday appealed the ruling by U.S. District Judge Robert Chatigny, who said testimony by a representative of one of Gramins's customers may have led jurors to mistakenly conclude that Gramins had a duty to tell the truth in negotiations.

Gramins's lawyer, Marc Mukasey, said Thursday that the "cases have proven to be a colossal failure for the government."